This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The glycogen storage diseases GSD are a set of genetic disorders in which the phenotype is severe hypoglycemia because of the inability to produce glucose from glycogen in the liver and kidney. Type I glycogen storage disease GSD1 is caused by a deficiency of hepatic glucose-6-phosphatase, which is an integral part of both glycogenolysis and gluconeogenesis. Lack of this enzyme results in the inability to maintain normal glucose homeostasis and severe fasting hypoglycemia. Patients presently are treated with an average of 6.2 doses of cornstarch per day. There is a need for a digestible starch that can provide a source of glucose for longer duration after meals in GSD patients. Vitastarch is a high amylase, heat modified waxy cornstarch which recently was found to have a lower glycemic index and be longer lasting when compared with traditional cornstarch in glycogen storage disease types 1a and 1b. A study is now proposed to determine the optimal dosing of this new product and to determine if modification of the amount of the new product will offer biochemical benefit and improved maintenance of glucose concentrations in patients with GSD 1a.